Interpreting PFAS Fate Through Thermal Hydrolysis To Support Biosolids Management Decision-Making

APPLICABILITY Growing regulatory scrutiny of per- and polyfluoroalkyl substances (PFAS) is increasing pressure on utilities to reassess biosolids management strategies in the absence of finalized regulatory thresholds or widely deployable treatment solutions. Evolving state requirements, uncertain federal timelines, reduced land application options, and incomplete understanding of PFAS fate through solids treatment processes complicate long-term planning for biosolids treatment, reuse, and disposal. In this environment, utilities are increasingly required to make consequential, risk-informed decisions using incomplete data, evolving analytical methods, and treatment configurations that were not originally designed with PFAS considerations in mind. Thermal hydrolysis has emerged as a key solids treatment technology for capacity expansion and process intensification, yet questions remain regarding how PFAS behavior through thermal hydrolysis and downstream anaerobic digestion should be interpreted and incorporated into broader biosolids management strategies.

DEMONSTRATED RESULTS AND OUTCOMES Methodology A full-scale evaluation of PFAS fate through solids treatment was conducted at Hampton Roads Sanitation District (HRSD) at the Atlantic Treatment Plant (ATP). The evaluation spanned an operating period characterized by conventional anaerobic digestion, followed by a post-consolidation period after implementation of thermal hydrolysis to provide additional solids treatment capacity required to support consolidation of wastewater flows at ATP following closure of another major treatment plant in the service area. The ATP-specific evaluation focused on changes in PFAS class distributions through solids treatment under differing operating conditions. In parallel, HRSD conducted broader biosolids PFAS characterization across additional treatment facilities to assess spatial variability and system-level implications for biosolids management. Together, these datasets were used to contextualize site-specific findings within a utility-wide decision-making framework under regulatory uncertainty. Results and Interpretation At ATP, anaerobic digestion during the pre-thermal hydrolysis operating period was associated with reductions in PFAS precursors and redistribution toward terminal PFAAs. Following implementation of thermal hydrolysis, observed PFAS class distributions in treated solids differed from those observed under digestion-only conditions (Figure 1). These differences reflected not only the change in treatment configuration, but also shifts in influent composition, internal recycle streams, and upstream flow routing associated with regional consolidation. Across operating periods, total fluorine concentrations exceeded fluorine attributable to quantified PFAS, highlighting the presence of uncharacterized organofluorine and reinforcing the limitations of PFAS-targeted analytical approaches when used in isolation. System-wide biosolids characterization further demonstrated substantial variability in PFAS concentrations among facilities within the same regional program (Figure 2), underscoring that PFAS-related risk is not uniformly distributed across a biosolids management system. Rather than indicating a single treatment outcome or technology effect, the results emphasize the importance of interpreting PFAS fate data within the broader operational and system context in which the data were generated. Conclusions/Outcomes Insights from the full-scale thermal hydrolysis evaluation, when considered alongside system-wide PFAS characterization and evolving regulatory context, were used to inform adjustments to biosolids management strategy while maintaining operational flexibility. Following release of the U.S. EPA's Draft Sewage Sludge Risk Assessment for PFOA and PFOS, biosolids routing and management approaches were revisited to balance PFAS-related risk considerations with capacity, reliability, and long-term planning needs (Figure 3). This work demonstrates that PFAS fate data are most valuable when interpreted cautiously, integrated across facilities, and viewed as one component of a broader decision-making framework. The presentation highlights how utilities can apply full-scale operational data to support adaptive biosolids management strategies without over-interpreting incomplete science or waiting for definitive regulatory endpoints.

RELEVANCE TO AUDIENCE This presentation will benefit utility managers, biosolids professionals, process engineers, planners, and regulators involved in solids treatment and biosolids management. The discussion is particularly relevant for organizations evaluating thermal hydrolysis or other solids process changes while navigating PFAS-related uncertainty. Rather than promoting a specific technology or prescriptive outcome, the presentation focuses on how PFAS fate data from full-scale operations can be responsibly interpreted, contextualized, and incorporated into practical biosolids management decisions under real-world constraints. The lessons presented are broadly applicable to utilities facing similar challenges associated with influent variability, evolving regulations, and uncertainty surrounding PFAS behavior in solids treatment systems.